In general, a spark plug includes an insulator having an axial hole extending along the axis thereof, a center electrode inserted into the axial hole, a metallic shell provided around the insulator, and a ground electrode which is provided at a front end portion of the metallic shell and which forms a spark discharge gap in cooperation with the center electrode. When a predetermined voltage is applied to the center electrode, spark discharge occurs at the spark discharge gap, whereby an air-fuel mixture is ignited.
Incidentally, as a result of use of such a spark plug, deposit such as carbon may adhere to the surface of the insulator. When accumulation of such deposit on the surface of the insulator proceeds, anomalous spark discharge along the surface of the insulator (so-called lateral flying spark) may occur between the center electrode and the metallic shell. In order to restrain occurrence of anomalous discharge, there has been proposed a technique of providing a space-forming portion of relatively small diameter at the front end of the center electrode to thereby form an annular space (so-called thermo pocket) between the space-forming portion and the wall surface of the axial hole, the annular space being open frontward with respect to the axial direction (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-49207).
Also, in recent years, reducing the diameter of a spark plug (metallic shell) is demanded for the purpose of, for example, increasing the degree of freedom of engine layout. However, merely reducing the diameter of only the metallic shell may result in excessive closeness between the metallic shell and the insulator. Therefore, the above-mentioned anomalous discharge, such as lateral flying spark, becomes more likely to occur. In view of such a problem, the wall thickness of the insulator may be reduced in order to secure a sufficiently large distance between the metallic shell and the insulator.
Problem to be Solved by the Invention
However, in a spark plug having a space-forming portion provided on its center electrode, electric field intensity increases at a boundary portion between the space-forming portion and a portion extending rearward from the rear end of the space-forming portion. Therefore, concentration of electric field intensity occurs in a region between the boundary portion and a front end portion of the metallic shell (particularly, a corner portion between the front end surface and inner circumferential surface of the metallic shell, at which electric field intensity is high). In the case where the insulator has a reduced wall thickness, penetration of discharge through the insulator may occur.
In recent years, there has been developed a combustion apparatus which utilizes a higher degree of compression and a higher degree of super charging in order to prevent drop in output while improving fuel consumption. However, in such a combustion apparatus, since a higher voltage is required for causing spark discharge, penetration of discharge through the insulator becomes more likely to occur. Moreover, in a lean burn engine or the like, since an air fuel mixture is lean (fuel concentration is low), an effect of cooling of a front end portion of the insulator by means of evaporation of the fuel becomes low, and the insulator becomes more likely to reach a higher temperature. Such overheating may lower the dielectric strength of the insulator, whereby penetration of discharge through the insulator may become more likely to occur.